In 870, Charles the Bald, King of West Francia, married Boso's sister Richilde. This marriage paved the way for Boso's career in the service of his royal brother-in-law. In the same year, Boso was appointed Count of Lyon and Vienne, replacing Gerard of Roussillon. In 872, Charles appointed him chamberlain and magister ostiariorum to his heir Louis the Stammerer. Boso likewise received investiture as Count of Bourges. Louis was reigning as a subordinate king of Aquitaine, but because of his youth, it was Boso who looked after the administration of that realm. In the autumn of 875, Boso accompanied Charles on his first Italian campaign and at the diet of Pavia in February 876 he was appointed arch-minister and missus dominicus for Italy and elevated to the rank of duke. He became Governor and Count of Provence in 877. He acted as a viceroy and married Ermengarde of Italy, the only daughter of the Emperor Louis II.
Out of favour
Boso disapproved of Charles' second Italian campaign in 877 and conspired with other like-minded nobles against his king. After Charles's death in October, these nobles forced Charles's son to confirm their rights and privileges. Boso also formed close relations to the papacy and accompanied Pope John VIII in September 878 to Troyes, where the Pope asked King Louis for his support in Italy. The Pope adopted Boso as his son and probably offered to crown Louis emperor. It is said that he wanted to crown Boso emperor.
Independent rule
In April 879, Louis the Stammerer died, leaving two adult sons, Louis III of France and Carloman II. Boso joined with other western Frankish nobles and advocated making Louis III of France the sole heir of the western kingdom, but eventually both brothers were elected kings. Boso renounced allegiance to the brothers and in July claimed independence by claiming the title Dei gratia id quod sum: by the Grace of God, that is what I am. He also claimed that his imperial father-in-law had named him as his heir. On 15 October 879, the bishops and nobles of the region around the Rhône and Saône rivers assembled in the Synod of Mantaille. They elected Boso King and successor to Louis the Stammerer, the first non-Carolingian king in Western Europe in more than a century. This event was the first "free election" among the Franks, without regard to royal descent, inspired by a canonical principle of ecclesiastical elections. Boso's realm, usually called the Kingdom of Provence, comprised the ecclesiastical provinces of the archbishops of Arles, Aix, Vienne, Lyon, and probably Besançon, and the dioceses of Tarentaise, Uzès, and Viviers. After Louis and Carloman divided their father's realm at Amiens in March 880, the two brothers joined to march against Boso. They took Mâcon and the northern parts of Boso's realm. Then uniting their forces with those of Charles the Fat, they unsuccessfully besieged Vienne from August to November. In August 882, Boso was again besieged at Vienne by his brother, Richard, Duke of Burgundy, Count of Autun, who took the city in September. Boso never regained much of his realm and was restricted to the county of Vienne. He died in 887 and was succeeded by his son Louis the Blind.
Marriages and issue
Boso was married twice. The identity of his first wife is not known; his second wife was Ermengard of Italy, only daughter of Louis II, Holy Roman Emperor, whom he wed in March 876. His issue was, in supposed chronological order:
Guilla/Willa, married firstly Rudolph I of Burgundy, secondly Hugh of Italy; her mother is reported to have been Ermengard, but this seems to be erroneous because she was born before Boso and Ermengarde's wedding
Ermengarde/Ermengard, married Manasses I the Old, Count of Chalons-sur-Seine ; her mother is reported to have been Ermengard
Engelberge/Ethelberga, married first Carloman II, secondly William the Pious; her mother is reported to have been Ermengard
Louis the Blind, possibly had a non-marital relationship, next supposedly married Anna/Eudocia Mamikonian, lastly married Adelaide of Burgundy, who might have been a daughter of his sister Guilla/Willa; his mother is reported to have been Ermengard
